Retroreflective materials are characterized by the ability to redirect light incident on the material back toward the originating light source. This property has led to the widespread use of retroreflective sheeting for a variety of traffic and personal safety uses. Retroreflective sheeting is commonly employed in a variety of articles, for example, road signs, barricades, license plates, pavement markers and marking tape, as well as retroreflective tapes for vehicles and clothing.
Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as “beaded” sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Due to the symmetrical geometry of beaded retroreflectors, microsphere based sheeting exhibits the same light return regardless of orientation, i.e., when rotated about an axis normal to the surface of the sheeting. For this reason, it is said that the distribution of light returned by beaded retroreflective sheeting is generally rotationally symmetric. Thus when viewing or measuring the coefficient of retroreflection (expressed in units of candelas per lux per square meter or Ra) at presentation angles from 0 to 360 degrees, or when measuring at orientation angles from 0 to 360, there is relatively little variation in the retroreflectivity of beaded sheeting. For this reason, such microsphere-based sheeting has a relatively low sensitivity to the orientation at which the sheeting is placed on a surface. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
Cube corner retroreflective sheeting, sometimes referred to as “prismatic” sheeting, typically comprises a thin transparent layer having a substantially planar first surface and a second structured surface comprising a plurality of geometric structures, some or all of which include three reflective faces configured as a cube corner element. Cube corner retroreflective sheeting is commonly produced by first manufacturing a master mold that has a structured surface, such structured surface corresponding either to the desired cube corner element geometry in the finished sheeting or to a negative (inverted) copy thereof, depending upon whether the finished sheeting is to have cube corner pyramids or cube corner cavities (or both). The mold is then replicated using any suitable technique such as conventional nickel electroforming to produce tooling for forming cube corner retroreflective sheeting by processes such as embossing, extruding, or cast-and-curing. U.S. Pat. No. 5,156,863 (Pricone et al.) provides an illustrative overview of a process for forming tooling used in the manufacture of cube corner retroreflective sheeting. Known methods for manufacturing the master mold include pin-bundling techniques, direct machining techniques, and techniques that employ laminae. These microreplication processes produce a retroreflective sheeting with prismatic structures that have been precisely and faithfully replicated from a microstructured tool having a negative image of the desired prismatic structure.
Prismatic retroreflective sheeting, in contrast to beaded retroreflective sheeting, is generally rotationally asymmetric. Thus when viewing or measuring Ra at presentation angles from 0 to 360 degrees, or when measuring at orientation angles from 0 to 360, there is significant variation in the retroreflectivity of prismatic sheeting. For this reason, prismatic sheeting has a higher sensitivity to the orientation at which the sheeting is placed on a surface than beaded sheeting.
Modification of prismatic sheeting by canting cubes is described, for example, in U.S. Pat. No. 4,588,258 (Hoopman). When the cubes along one groove are canted, retroreflectivity generally increases at larger entrance angles along an axis perpendicular to that groove. Retroreflectivity tends to decrease at larger entrance angles at orientations that are not close to the axis perpendicular to that groove. Thus canted sheeting tends to have increased variation in retroreflectivity at a given orientation angle. For this reason, canted sheeting is especially rotationally asymmetric.
As is stated in U.S. Patent Publication No. 2009/0142486 (Hannington), “a demand exists for retroreflective materials having discernible patterns, graphics, or validation images formed thereon.” Beaded sheeting having specific graphic images or marks has been used on license plates to act as a means of verifying the authenticity or valid issuance of the license plate. For example, license plates in Washington, D.C. have generally included an identifying mark imprinted in reflective sheeting since 1986. The security mark is round and appears in a repeating pattern down the center of the license plate (as evident from http://dcplates.com/Glossary.htm). The security mark can be seen clearly only when the plate is viewed at a 30 degree angle, and the mark is placed in the sheeting by its manufacturer for control purposes during the production process.
Another security mark for use on license plates using beaded sheeting is described, for example, in U.S. Pat. No. 7,068,434 (Florczak et. al.). This security mark is formed in beaded sheeting as a composite image that appears to be suspended above or below the sheeting. Because of its appearance, this type of security mark is generally referred to as a floating image.
Other types of beaded sheeting including security marks include those described, for example, in U.S. Patent Publication No. 2009/0142486 (Hannington) (relating to the inclusion of a layer of transparent microsphere lenses embedded in a spacing layer to form an image) and U.S. Pat. No. 4,634,220 (Hockert) (relating to laser irradiation of the front surface of the sheeting to form an image).